1. Field of the Invention
The present invention relates to the mounting of electronic chips on one another to form so-called three-dimensional integration structures.
2. Discussion of the Related Art
Generally, an electronic system comprises several electronic chips, for example, processors and memories. To connect these chips, it is usual to mount these chips on the same substrate and to connect each of the chips to the substrate.
To decrease the occupied surface area and make the assembly more reliable, so-called three-dimensional structures have been provided, in which a first surface-mounted chip is directly mounted on a second chip to which it is connected.
Since each of the two chips may be associated with a different substrate, for example made of glass, silicon or gallium arsenide, the connection of the two chips becomes difficult. Indeed, the chips may have different mechanical properties. Further, the connection pads of the two chips may be sized differently. The electric connection between the two chips is thus made difficult.
Further, the increase of the transistor density in a circuit and the increase of the transistor switching frequencies generate the occurrence of hot spots in the chips. The heat flows may be greater than 300 W/cm2. If a hot spot appears in the first chip, the thermal radiation on the second chip may deteriorate the latter.